Fluorescent lamps are used as light sources in a wide variety of applications. These applications include consumer and industrial applications, such as home and office lighting. Fluorescent lamps are also used in a number of more demanding applications. For example, fluorescent lamps are used in backlights for displays, such as active matrix liquid crystal displays (AMLCD). From a weight point of view, when compared to cathode ray tube displays, AMLCDs are ideally suited for use in aerospace applications, such as primary flight instrument displays. Unfortunately, aircraft, particularly military aircraft, are often operated in extremely cold temperatures. Because extremely cold temperatures can affect a fluorescent lamp's performance, extremely cold temperatures can affect an AMLCD display that is backlit by a fluorescent lamp. The present invention is directed to reducing the effect of extremely cold temperatures on the performance of fluorescent lamps.
The electrical energy delivered to a fluorescent lamp is converted to visible light emission by mercury atoms. During the fabrication of a fluorescent lamp, liquid mercury is injected into a glass enclosure that defines a lamp wall. Depending upon the temperature of the fluorescent lamp wall, a fixed portion of the bulk mercury vaporizes and becomes part of the discharge gas mixture. Most of the discharge gas mixture is formed by a single rare gas, such as argon, or a mixture of rare gases, such as neon and argon. The rare gas atoms act as a buffer and produce little useful light output.
The mercury atoms are excited to upper energy levels by collisions with energetic electrons in the discharge gas mixture. Some of the excited mercury atoms emit UV radiation while returning to their ground state. The UV radiation activates a phosphor coating on the interior of the fluorescent lamp wall that produces visible light. The magnitude of the visible light output of the fluorescent lamp is determined by the mercury pressure, which is proportional to the temperature of the fluorescent lamp. The visible light output of the fluorescent lamp is maximized at an optimum temperature and corresponding mercury pressure.
FIG. 1 shows that, for a fluorescent lamp having an enclosure with a small diameter, such as 15 mm, the optimum temperature is about 50.degree. C. If the temperature is below the optimum temperature, mercury atoms condense onto the lamp wall or other cold internal surface, such as filament leads, and the UV radiation production rate is reduced. This decreases the visible light output from the fluorescent lamp. Raising the temperature above the optimum temperature leads to further increases in the mercury atom concentration. This causes radiation trapping, or imprisonment of the UV light, and a corresponding decrease in lamp efficiency.
In some environments, such as military aircraft, it is desirable that primary flight instruments reach flight readiness within one to ten minutes. This means that the displays associated with such instruments also become flight ready within this period of time. In the case of an AMLCD backlit by a fluorescent lamp, this means that the fluorescent lamp must reach peak output in a short time period (0.5 to 2 minutes). In the past, this has been difficult, if not impossible, to achieve when the military aircraft is located in a low-temperature region, such as the Arctic.
At low temperatures, mercury vapor condenses on the lamp wall and/or the electrodes of a fluorescent lamp. As a result, the visible light output of the fluorescent lamp is restricted by the warmup rate of the mercury within a fluorescent lamp. It is known in the art to accelerate the warmup rate of the mercury within the fluorescent lamp by passing an electrical current through a small diameter wire wrapped around the exterior of the fluorescent lamp. Even with this modification, it may take several minutes for the wire, acting as a heater, to raise the temperature of the glass enclosure to the temperature and corresponding mercury pressure at which the peak light output of the fluorescent lamp is maximized. This time depends upon available heater power and ambient temperatures.
It is also known to increase the concentration of mercury atoms in a fluorescent lamp by attaching an amalgam material to the electrode assembly of the fluorescent lamp. The amalgam material forms an alloy with the mercury. When the electrode is heated prior to initiation of the gas discharge, the mercury is released by the amalgam attached to the electrode. Excess mercury is thus introduced into the discharge gas mixture and the fluorescent lamp reaches peak luminosity almost instantaneously. Unfortunately, the mercury in the amalgam material is depleted within one to two minutes. If the entire fluorescent lamp does not reach its optimum operating temperature within this time period, mercury liberated from the amalgam material will condense at the coldest spot within the fluorescent lamp. The visible light output from the fluorescent lamp is reduced once the mercury content of the amalgam is exhausted.
The use of a silver amalgam in the electrode assembly of fluorescent lamps designed for consumer applications to improve the cold-start performance has been suggested. See The Journal of the Illuminating Engineering Institute of Japan, Vol. 68, No. 10, October 1984, pp. 524-527. The intended application is an energy-saving fluorescent replacement for a standard, screw-in type incandescent lamp. Other amalgam materials, such as indium, bismuth-indium and lead-tin-bismuth, also have been used to improve the visible light output of a fluorescent lamp at startup over a wide range of temperatures. See Bloem, Bouwknegt and Wesselink, Journal of the Illuminating Engineering Society, April 1977, pp. 141-147. Unfortunately, at low temperatures, an indium amalgam releases too much mercury. This excessive release interferes with fluorescent lamp ignition. It also causes mercury to deposit on and blacken the ends of a fluorescent lamp wall.
The use of two different amalgam compositions to regulate mercury pressure at low and high temperatures is also known. A dual amalgam combination allows a fluorescent lamp to operate over a wider temperature range than does a single amalgam. Because a dual amalgam approach suffices for fluorescent lamps designed for consumer applications that are not subject to a wide temperature range, further luminosity control measures are neither necessary nor cost effective. Unfortunately, a dual amalgam cannot regulate mercury pressure throughout the range of ambient temperatures encountered in aerospace and military applications.
In addition to the low-temperature performance deficiencies discussed above, some conventional amalgam materials are difficult to use in some fluorescent lamps because of manufacturing requirements. For example, in order to improve usable life, manufacturing specifications require that the entire fluorescent lamp structure of serpentine lamps, of the type typically used for backlighting avionics displays, be heated to several hundreds of degrees centigrade during manufacturing. These temperatures are well above the melting point of many common amalgam materials. Indium's melting point is 157.degree. C. The inclusion of an amalgam with a low melting point entails the use of processing methods that are more time consuming and complex than are the processing methods used when the chosen amalgam materials have high melting points. Further, lower fluorescent lamp processing temperatures can shorten the lifetime of fluorescent lamps by not sufficiently baking out impurities.
It is also known that the light output of a fluorescent lamp can be held constant after warmup by controlling the temperature of a spot on the wall of a fluorescent lamp using a solid state cooling device, such as a Peltier cooler. See U.S. Pat. Nos. 3,309,565 and 4,529,912, for example. As discussed in these patents, in the past, the use of Peltier devices to maintain the output of fluorescent lamps at a desired level as ambient temperature varies has been suggested. However, because Peltier devices are costly, this technology has not been implemented. Rather, dual amalgam combinations have been widely used as low-cost alternatives to Peltier devices. Neither approach has been used to decrease the warmup of fluorescent lamps designed for use in low-temperature climates.
The present invention is directed to providing a fluorescent lamp that is ideally suited for use in the backlight of AMLCDs designed for military aircraft and other displays intended to be operable in low-temperature conditions that overcomes the foregoing and other disadvantages of fluorescent lamps intended to be operable in such conditions. While designed for use in the backlight of AMLCDs intended for use in military aircraft displays, it is to be understood that fluorescent lamps formed in accordance with the present invention may also find use in other environments, including other types of military vehicles.